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There are two types of low temperature stress: chilling and freezing. Chilling injury is sustained by different species over the range from 20 to 0oC. The resultant injuries may include a variety of physiological disruptions in germination, flower and fruit development, yield, and storage life. Minor chilling stress at non-lethal temperatures is normally reversible. Exposure to gradually decreasing temperatures above the critical range can result in hardening of plants that may reduce or eliminate injury during subsequent exposure to low temperatures. Most species are not injured until the temperature drops below zero, suffering only a temporary inhibition of growth and metabolism, from which they can recover when warmed. However crops of tropical origin as well as many of subtropical origin are sensitive to chilling at temperatures well above zero, from 4oC for temperate fruits, 8oC for subtropical fruits, and about 12oC for tropical fruits. Amongst the highest volume world food crops, maize and rice are sensitive to chilling temperatures. Their growth and development can be adversely effected by temperatures below 10oC resulting in yield loss or crop failure. Chilling also affects important crops like cotton, soybean and tomato.
Treatments for reproducing chilling or cold stress should take into account the type of species, the rate at which the temperature decreases, and the period of time at which above-lethal temperature is maintained.
The second type of injury is freezing injury. This type of injury occurs when the external temperature drops below the freezing point of water in the tissue (which can be significantly lower than zero). Plants may experience intracellular freezing and/or extracellular freezing. Intracellular freezing damages the protoplasmic structure and the ice crystals kill the cell once they grow large enough to be detected microscopically. In extracellular freezing, the protoplasm of the plant becomes dehydrated because a water-vapor deficit is created as cellular water is transferred to ice crystals forming in the intercellular spaces. In some cases, water can remain liquid as low as -47oC without nucleating and forming ice. When nucleation of this supercooled water does occur, intracellular ice forms suddenly resulting in death of the plant.
The reproduction of a natural freezing stress for intact plants is extremely difficult in a growth chamber, even in darkness, as the gradient of cold in an enclosed chamber is opposite to that occurring naturally, the ground normally being warmer than the air. Protocols using detached tissues may overcome this problem.
Impact of Cold Stress. DB Fowler and AE Limin
Crop Development Centre, University of Saskatchewan, Saskatoon, Canada